A. Field of the Invention
This invention relates generally to the subject of precision optical imaging systems such as found in telescopes, aerial reconnaissance and surveillance cameras, advanced weapons, microscopes and other high performance applications, and more particularly to a methods and apparatus for characterizing the performance of such an optical system, and improved optical systems including such characterization apparatus. Characterization of the performance of an optical system has several applications. One such application, in the reconnaissance scenario, is increased accuracy of geolocation of targets in an image taken by the camera on the surface of the earth. Other applications include passive weapon aiming, passive determination of range from the imaging system to a target, improved mapping of the earth, and vehicle navigation from imagery of known objects.
B. Description of Related Art
Aspects of this invention have resulted from the present inventors' labors to improve the ability of an aerial reconnaissance camera system to geolocate targets in an image taken from the camera. Reconnaissance cameras are known in the art and described in the patent literature, see for example U.S. Pat. Nos. 5,155,597, 5,668,593 and 6,694,094, all assigned to the assignee of this invention.
Today's modern reconnaissance cameras typically use solid state image detector arrays made of semiconductor materials and composed of individual pixel elements arranged in a rectangular format of rows and columns. This disclosure takes advantage of the concept that a camera, at an elemental level, can be viewed as a device that translates incoming ray angles to positions, where the positions in the image have pixel addresses, on a focal plane array. In particular, when a target on the ground is imaged by an aerial reconnaissance camera, a ray in three-dimensional space extending from the camera to the target can be calculated, given the camera's position and orientation at the time of the exposure. The intersection of this ray on the focal plane array is, in theory, at a single point on the array for a suitably small target. The point where the ray intersects the array can be identified as a pixel with a given row and column address. With this information known, as well as the camera's position in space and its orientation at the time of the exposure of the array, it is possible using known algorithms to estimate the location of the target corresponding to this pixel address on the surface of the earth. The camera's position in space is typically obtained from an inertial measurement unit, that maybe associated with a Global Positioning System (GPS) unit included in a camera electronics package or from a similar navigation system present aboard the aircraft.
Object geolocation from imagery has uses in a variety of fields, including mapping, and in military applications, including surveillance and locating potential targets for precision guided munitions.
There are several complicating factors that must be addressed to reach desired levels of geolocation accuracy with current operational reconnaissance cameras. These complicating factors have not been satisfactorily addressed in the known prior art. Two primary factors are as follows:
1) Optical distortion of the camera exists, and changes with time, in ways that are not easily predicted. For example, a camera may have many mirrors and or lenses in its optical path, and a focusing element, all of which can introduce significant distortion. Such distortions are influenced by temperature, pressure, humidity, and also by mechanical factors in the camera's design and construction, including the effects of vibration component misalignment, and the expansion or contraction of materials. Other changes may be deliberate, such as adjustment or other optical compensation.
2) All cameras have a line of sight that represents the direction that the camera is imaging. The line of sight is typically the center of the field of view of a camera. The direction of that line of sight may be governed by the orientation of the camera, the orientation of a gimbaled portion of the camera, a pointing mirror, or other means. Some cameras have structures that maintain a fixed relationship to the line of sight, while others do not. The structure may be too small to support an inertial measurement unit. Therefore a means for measuring the position of this line of sight or for determining the effect of some gimbal, mirror, or other optical elements on the camera field of view is required. Furthermore, geolocation requires periodic measurement of camera orientation during operation (i.e., measurement as every image is captured), and the camera line of sight or field of view needs to be related to geographic coordinates. Data as to both camera position & camera attitude are required.
The methods and apparatus disclosed herein address both of these problems. The solutions disclosed herein are also applicable to optical systems generally.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.